They're selective because liberal commentators don't apply the same basic progressive standards to their approved candidates. I absolutely agree that the points you've made about Gabbard are legitimately bad and I wouldn't vote for her. But Howard Dean remains a Democrat in good standing despite his lobbying for the MEK organization. Countless Democrats including Hillary Clinton take money from the Saudi Kingdom. Even on the marriage equality question, we're talking about a party that only came around on this issue in roughly 2014. If the liberals attacking Gabbard actually applied their principles consistently, American politics would look a lot different. But they don't. And we should investigate why they don't.
Relevant democrats like...Howard Dean. Yeah it's a real mystery why we don't see more talk about him.